1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp to illuminate a liquid crystal display device, and particularly to a discharge lamp lighting apparatus provided with a function of detecting an abnormal electrical discharge.
2. Description of the Related Art
An illumination device such as a backlight device is used in a liquid crystal display (LCD) as a display device for a liquid crystal monitor, an LCD television, and the like. A discharge lamp such as a cold cathode discharge lamp is extensively used as a light source for such an illumination device, and a discharge lamp lighting apparatus usually includes an inverter circuit provided with a step-up transformer to achieve a high AC voltage required to duly light the discharge lamp.
Conventionally, an inverter circuit for a discharge lamp lighting apparatus includes a high voltage capacitor connected at the secondary side of a transformer, and a resonant circuit is formed by the high voltage capacitor together with a leakage inductance of the transformer and a parasitic capacitance of the discharge lamp connected to the transformer as a load, wherein the primary side of the transformer is driven at the resonant frequency of the resonant circuit (refer to, for example, U.S. Pat. No. 6,114,814).
FIG. 7 is a circuitry of an example of such a discharge lamp lighting apparatus as described above. A discharge lamp lighting apparatus 50 shown in FIG. 7 includes a transformer 54 which has its primary winding connected to output terminals 51a and 51b of an H-bridge circuit (not shown), and which has its secondary winding connected to a discharge lamp 56 via a resonant circuit 59 which is composed of a leakage inductance of the transformer 54, a high voltage capacitor 58, and a parasitic capacitance (not shown) of the discharge lamp 56. In the discharge lamp lighting apparatus 50, the operating frequency of the H-bridge circuit to drive the primary side of the transformer 54 is set to the resonant frequency of the resonant circuit 59 so that the power efficiency of the transformer 54 can be enhanced.
Since an inverter circuit generally outputs a high voltage, abnormal electrical discharges can occur at the current route (including a discharge lamp) carrying an AC output from the inverter, such as: an arc discharge caused due to breakage of circuit wirings, for example, cracking at a soldered portion, defective connection at a connector, or deformation of a component or wire by an external force; a breakdown discharge found between high-voltage and low-voltage portions; and a ground discharge. An arc discharge, for example, is accompanied by sparks, which may possibly damage terminals or components, or may even give off smoke or fire. In order to address such a problem found at a discharge lamp lighting apparatus provided with a step-up transformer, there is provided a circuit to detect an abnormal discharge and also stop supply of electric power to the discharge lamp thereby preventing damages to the discharge lamp lighting apparatus and the LCD device (refer to, for example, Japanese Patent Application Laid-Open No. 2005-183099).
FIG. 8 is a block diagram of an example of such a discharge lamp lighting apparatus. Referring to FIG. 8, a discharge lamp lighting apparatus 100 includes a transformer 105, a transformer driving circuit 104 connected at the primary side of the transformer 105, and a control circuit 103 connected to the transformer driving circuit 104 and adapted to control the operation of the transformer driving circuit 104. A discharge lamp 106 is connected via its one terminal to one terminal of the secondary winding of the transformer 105 and via its other terminal to a current-voltage converting circuit 107 to convert a lamp current into a voltage. The output from the current-voltage converting circuit 107 is inputted to the control circuit 103 via a lamp current controlling pattern 108, and the control circuit 103 controls the transformer driving circuit 104 according to the output signal so as to make the lamp current stay constant. A discharge detecting pattern 111 is connected at the other terminal (ground side) of the secondary winding of the transformer 105 and arranged so as to go along and close to the lamp current controlling pattern 108.
In the discharge lamp lighting apparatus 100 described above, if a corona or arc discharge is caused at a breakage in the wiring at the secondary side of the transformer 105, a noise component is mixed into the lamp current. Due to a high frequency component included in the noise component, an induced voltage is generated in the discharge detecting pattern 111 disposed along and close to the lamp current controlling pattern 108, and is inputted to the control circuit 103 via a discharge detecting diode 112 and an integration circuit 113. Then, the control circuit 103 compares the inputted voltage with a reference voltage predetermined, and if the inputted voltage exceeds the reference voltage, the transformer driving circuit 104 is caused to stop its operation.
Thus, in the discharge lamp lighting apparatus 100, a corona or arc discharge, when caused in the circuits of the transformer 105, is duly detected, and power supply to the secondary side of the transformer 105 is disconnected to thereby stop discharging so that the discharge lamp lighting apparatus 100 and the LCD device can be protected.
In the discharge lamp lighting apparatus 50 of FIG. 7, the high voltage capacitor 58, which is a relatively costly capacitor with a high withstand voltage, is connected at the secondary side of the transformer 54, thus inviting a cost increase problem. Since a large LCD for an LCD television incorporates a backlight device using a plurality of discharge lamps in order to achieve a high brightness, the high-voltage capacitor 58 must be provided in a number corresponding to the number of discharge lamps used, which aggravates the cost increase problem.
In order to cope with the problem, a pattern capacitor, which is composed of a board as a dielectric body and electrode patterns formed on the board, may be used in place of discrete electronic components for the high voltage capacitor 58. However, in a discharge lamp lighting apparatus like the discharge lamp lighting apparatus 50 of FIG. 7, in which the transformer 54 is driven at the resonant frequency of the resonant circuit 59 (or at a specific frequency predetermined in relation to the resonant frequency), the following problem is raised in association with the usage of the pattern capacitor.
The parasitic capacitance value of the discharge lamp 56, which is affected by the distance between the discharge lamp 56 and a metal chassis having the discharge lamp 56 attached thereto, is caused to vary due to a change in the design of the metal chassis or the structure for attaching the discharge lamp 56 to the metal chassis, and accordingly the resonant frequency of the resonant circuit 59 is also caused to vary. So, when such a pattern capacitor as described above is used in place of the high-voltage capacitor 58, a design change must be implemented on the pattern capacitor according to the variation of the parasitic capacitance value of the discharge lamp 56. Consequently, whenever the parasitic capacitance of the discharge lamp 56 is changed, the pattern capacitor used as the high-voltage capacitor 58 must undergo a design change, that is to say a design change must be implemented on a circuit board, which generally requires time and cost.
Further, the discharge lamp lighting apparatus 50 also desirably has a function of detecting abnormal discharges as provided in the discharge lamp lighting apparatus 100 described with reference to FIG. 8. The function of detecting abnormal discharges in the discharge lamp lighting apparatus 100, however, is provided such that the discharge detecting pattern 111 is disposed at the ground side of the secondary side of the transformer 105 therefore failing to directly detect the high voltage portion where a discharge phenomenon is actually caused, and thus the detection accuracy is not satisfactory.